Translation:Animals are ill.
I don't think "Animals are ill" should be encouraged by Duolingo as another correct option. If the article "the" is left out, the sentence implies that all animals are sick and we know that is not true. Whereas, "The animals are ill," signifies a certain group of animals is sick.
은/는 are topic markers and 이/가 are subject markers. Topic markers can be seen as meaning the word "A" and subject markers can be seen as meaning the word 'The'. But that doesn't mean that Korean won't change those meanings when ever the language feels like it.
은 and 이 are used after words that end in a consonant while 는 and 가 are used after words that end in vowels.
There are many places on the internet you can find that explain the topic and subject markers, none of which make sense to me. Talk To Me In Korean has YouTube videos explaining the uses for topic/subject markers.